LCDs are widely used in various electronic information products, such as notebooks, personal digital assistants, video cameras, and the like.
FIG. 4 is an abbreviated circuit diagram of a conventional LCD. The LCD 10 includes a scanning circuit 110, a data circuit 120, a power supply circuit 130, and a liquid crystal panel 140. The liquid crystal panel 140 includes n rows of parallel scanning lines 141 (where n is a natural number), m columns of parallel data lines 142 perpendicular to the scanning lines 141 (where m is also a natural number), and a plurality of pixel units 148 cooperatively defined by the crossing scanning lines 141 and data lines 142. The pixel units 148 are arranged in a matrix. The scanning lines 141 are connected to the scanning circuit 110, and the data lines 142 are connected to the data circuit 120.
Each pixel unit 148 includes a thin film transistor (TFT) 143, a pixel electrode 144, and a common electrode 145. A gate electrode of the TFT 143 is connected to a corresponding one of the scanning lines 141, and a source electrode of the TFT 143 is connected to a corresponding one of the data lines 142. Further, a drain electrode of the TFT 143 is connected to the pixel electrode 144. The common electrode 145 is generally opposite to the pixel electrode 144, with liquid crystal molecules (not shown) sandwiched therebetween, so as to cooperatively form a liquid crystal capacitor 147.
Referring to FIG. 5, the power supply circuit 130 includes a control unit 131, a first transistor 132, a second transistor 133, a resistor 134, and an output terminal 135. The first and second transistors 132, 133 are both P-channel metal oxide semiconductor (PMOS) transistors. A gate electrode of the first transistor 132 is connected to the control unit 131, and a drain electrode of the first transistor 132 is configured to receive a power voltage signal VCC. Further, a source electrode of the first transistor 132 is connected to the output terminal 135, and also connected to a drain electrode of the second transistor 133. A gate electrode of the second transistor 133 is connected to the control unit 131, and a source electrode of the second transistor 133 is grounded via the resistor 134. The output terminal 135 is further connected to the scanning circuit 110.
In operation, the scanning circuit 110 provides a plurality of scanning signals to the scanning lines 141 sequentially, so as to activate the pixel units 148 row by row. The data circuit 120 provides a plurality of data voltage signals to the pixel electrodes 144 of the activated pixel units 148. Thereby, the liquid crystal capacitors 147 of the activated pixel units 148 are charged, and an electric field is generated between the pixel electrode 144 and the common electrode 145 in each pixel unit 148. The electric field drives the liquid crystal molecules to control light transmission of the pixel unit 148, such that the pixel unit 148 displays a particular color (red, green, or blue) having a corresponding gray level. The aggregation of colors displayed by all the pixel units 148 simultaneously constitutes an image viewed by a user of the LCD 10.
When the LCD 10 is switched off, an external command is provided to the control unit 131 of the power supply circuit 130, and the control unit 131 correspondingly provides a low level voltage signal to switch the first transistor 132 on, and provides a high level voltage signal to switch the second transistor 133 off. Thereby, the power voltage signal VCC is outputted to the scanning circuit 110 via the first transistor 132. Due to the power voltage signal VCC, the scanning circuit 110 provides high level voltage signals to all the scanning lines 141 simultaneously, such that all the TFTs 143 are switched on, and the liquid crystal capacitors 147 are discharged. After the discharging process, the electric field in each pixel unit 148 is removed, and the image displayed by the LCD 10 disappears.
However, the external command may not last for a sufficiently long period of time. If the external command lapses within the discharging process, the control unit 131 is liable to stop providing the low level voltage signal to the first transistor 132. In this circumstance, the power voltage signal VCC cannot output to the scanning circuit 110, and accordingly the high level voltage signals outputted by the scanning circuit 110 are canceled. Thus the discharging process stops ahead of time, and the liquid crystal capacitors 147 are incapable of discharging completely. Residual charges in the liquid crystal capacitors 147 may cause an unwanted residual image to be displayed on the LCD 10.
What is needed is to provide an LCD that can overcome the above-described deficiencies.